The present invention generally relates to the production of corrugated cardboard, and more particularly, to a novel and improved method and apparatus for accurately applying an adhesive to the flutes of corrugated board so that the flutes can be bonded to a face.
Typically, corrugated cardboard is formed by producing a corrugated sheet which is initially bonded along one side to a single face. Adhesive is then applied to the crests of the flutes remote from the single face by an applicator roll of a glue machine. Thereafter, a second face is applied to the adhesive on the flutes to produce a composite structure in which corrugations extend between and are bonded to spaced-apart faces.
In some instances, multiple-layer cardboard is produced in which more than one corrugated sheet is adhesively attached to additional faces so that, for example, a central flat face is bonded to a corrugated sheet on each side thereof, and outer flat faces are bonded to the sides of the two corrugated sheets remote from the central face.
The corrugated sheet is typically passed between a rider roll and an applicator roll to apply the adhesive to the flutes. The rider roll typically applies sufficient downward pressure to force the flute tips into contact with the applicator roll. This downward pressure causes compression or deformation of the flutes. The flutes enter the adhesive layer prior to being crushed against the applicator and often become overly wetted or saturated with adhesive due to the long dwell time. As a result, the flutes do not return to their original shape after being crushed. This permanent deformation of the flutes reduces the strength of the final cardboard.
The adhesive applied to the flutes is unsymmetrical because the flutes plow through the adhesive layer on the applicator and are wetted on one side more than the other. This unsymmetrical application of the adhesive results in a lower bond strength for a given weight of adhesive and an rough surface finish on the face sheet due to warpage after the adhesive cures. Additionally, a relatively large amount of over spray is created which further increases the amount of glue used by the process.
Accordingly, there is a need in the art for an improved method and apparatus for producing corrugated cardboard which obtains maximum strength in the finished product and an improved surface finish on the face. Furthermore, it is desirable to apply substantially less adhesive per unit area of the finished product and to produce the improved cardboard at an increased rate of production.